Membrane separation techniques are used in wide areas such as desalination of seawater and brackish water, production of medical and industrial pure water and ultrapure water, industrial wastewater treatment and food industry. In such membrane separation, the contamination of the separation device caused by microbes impairs the quality of the obtained permeating water, and furthermore promotes the growth of microbes on the membrane surface and the deposition of microbes and their metabolites on the membrane surface, to lower the permeability and separability of the membrane. To avoid these serious problems, various methods for sterilizing the membrane separation device are proposed, and generally, a microbicide is constantly or intermittently added to the feed liquid. As the microbicide, most generally a chlorine-based microbicide advantageous in view of price and operation is added to achieve a concentration of 0.1 to 50 ppm. Furthermore, an effective sterilization method, in which less expensive sulfuric acid is added to lower the pH of the liquid fed to the membrane separation device to 4 or less, is also developed (EP1031372A). As the piping of the membrane separation device, usually a corrosion resistant metal such as stainless steel is used, but if the addition of sulfuric acid or the like raises the acidity, since the metal goes into the corrosion region of the Pourbaix diagram, the piping is liable to be corroded. In a state where the acidity is low, there are such problems that the sterilization frequency must be increased and that longer sterilization time is necessary for enhancing the sterilization effect.
It would therefore be advantageous to overcome the above-mentioned disadvantages of the prior art by providing a water-treating microbicide and a water treatment method having a high sterilization effect.